You can face two types of offers:
those which show you the progress and pay out coins for each step achieved:
In that case you can always come back to the offerwall to check whether your progress is being tracked. If you see that for some stage you've completed you haven't received coins you need to immediately stop completing the offer and contact the offerwall!
those where you need to achieve one final goal to get the reward:
In that case you will receive coins only after the final goal is achieved.